When you send your sim on an adventure, she will often find canopic jars and pieces of sarcophagi. Collecting and building these is one way to add another sim to your family.
Collect enough sarcophagus fragments to build one, which is five fragments for any of the sarcophagus types, then collect five canopic jars of any type. Your sim must be on her home lot before you will get the reconstruct action for the sarcophagus; unfortunately, the base camp lot in any of the adventure towns will not work, but if your sim has purchased a vacation home, the sarcophagus can be built there. Once the sarcophagus is built, make sure the canopic jars are in your sim’s inventory and have her use the “summon mummy” interaction on the sarcophagus. She will put the jars into the sarcophagus and step back as the mummy appears. Once in a while this interaction will fail, but don’t worry, the jars that you tried to use will be restored in your sim’s inventory.
Voila! You now have a mummy in your household. The mummy has the usual hunger, social, hygiene and fun needs, but bladder and energy needs both appear with mummy wrapping around them. Hovering over the bladder need sows this tooltip:
If you happen to have the Sarcophagus of the Kings on your lot, you can have the mummy rest in it and he will soon be “cured” of his mummification and become a regular sim.